This invention relates generally to communications systems and, more particularly, to apparatus and methods for controlling crosspolarization of signals in a frequency reuse system in which dual polarized signals carry independent information at the same frequency.
In recent years, the demand for communications has grown tremendously, and an even more rapid growth is expected in the future. Both terrestrial communications systems and satellite communications systems have been improved and expanded to meet this demand. In the field of satellite communications, for example, because of its many advantages, even greater demand is placed on this form of communications. This has resulted in the allocated spectrum for satellite communications becoming more and more crowded.
In view of this demand, substantial efforts have been made to try to utilize the frequency spectrum more efficiently, particularly for satellite communications systems. One effort has resulted in a frequency reuse communications technique in which two signals having independent information share the same channel frequency. In other words, since two signals share a frequency, every frequency in the spectrum can be used twice, thereby expanding the capacity of the communication channels by a factor of two.
One way to achieve a frequency reuse technique is to employ orthogonal polarizations, that is, dual signals which are orthagonally polarized in relation to one another. Typically, the two signals are either linearly polarized, in which the signals are transmitted at right angles to one another, or are oppositesensed circularly polarized, in which the two signals rotate in opposite directions.
The feasability of the frequency reuse technique depends on the amount of discrimination which can be achieved between the two signals. For various reasons, during the transmission of the signals there will always be some amount of signal energy transferred from one polarization to the other. This energy transfer is called the crosspolarization effect, which will result in some level of interference in each of the two signals. The extent of this effect determines the performance of the dual-polarization system.
There are many sources in the communications link which will cause the crosspolarization effect. In transmitting and receiving systems generally, for example, the antennae, the wave guide, and the orthomode transducer can cause crosspolarization. In a satellite communications system, in the propagative medium, the rain, clouds, snow, etc., can cause crosspolarization. Among all the crosspolarization effects, rain-crosspolarization at microwave frequencies has been found to be the most serious problem. This is owing to the fact that the problems in the transmitting and receiving systems can be improved by carefully designing these systems. The effects of clouds and snow are negligible compared to the effect of rain drops, but the rainfall, of course, can not be controlled.
Many different systems have been designed to solve the rain-crosspolarization problem in satellite communications systems. While these systems are different, their basic approaches are all the same. Each receiving earth station in the satellite communications system receiving a transmission of dual-polarized signals from one transmitting earth station, attempts to cancel the crosspolarization in the received signals induced by any rain at both the transmitting and receiving end. More particularly, a receiving station will have a network which is set or adjusted to cancel the crosspolarization of signals being received from the one transmitting station in the system.
A problem with the above cancellation system is that if a receiving or local station is intended to receive, simultaneously, fifty dual-polarized signals transmitted by fifty different transmitting or remote stations located around the world, then fifty such networks are necessary to cancel, respectively, the crosspolarization in the fifty dual-polarized signals being received. This is because it is not unlikely that the rain pattern at many, if not all, of the transmitting stations, may be different from one another. The different rain patterns produce different rain-crosspolarization effects, which means that the dual-polarized signals being propagated through the rain around one transmitting station will be crosspolarized differently than the dual-polarized signals being propagated through the rain around another transmitting station. Consequently, the receiving station will require the fifty different networks, each of which will be adjusted to cancel the crosspolarization of signals being received from a corresponding transmitting station.